The essential amino acid Tryptophan (Trp) is catabolized through the kynurenine (KYN) pathway. The initial rate-limiting step in the kynurenine pathway is performed by heme-containing oxidoreductase enzymes, including tryptophan 2,3-dioxygenase (TDO), indoleamine 2,3-dioxygenase-1 (IDO1), and indoleamine 2,3-dioxygenase-2 (IDO2). IDO1 and IDO2 share very limited homology with TDO at the amino acid level and, despite having different molecular structures, each enzyme has the same biochemical activity in that they each catalyze tryptophan to form N-formylkynurenine. IDO1, IDO2, and/or TDO activity alter local tryptophan concentrations, and the build-up of kynurenine pathway metabolites due to the activity of these enzymes can lead to numerous conditions associated with immune suppression.
IDO1 and TDO are implicated in the maintenance of immunosuppressive conditions associated with the persistence of tumor resistance, chronic infection, HIV infection, malaria, schizophrenia, depression as well as in the normal phenomenon of increased immunological tolerance to prevent fetal rejection in utero. Therapeutic agents that inhibit IDO1, IDO2, and TDO activity can be used to modulate regulatory T cells and activate cytotoxic T cells in immunosuppressive conditions associated with cancer and viral infection (e.g. HIV-AIDS, HCV). The local immunosuppressive properties of the kynurenine pathway and specifically IDO1 and TDO have been implicated in cancer. A large proportion of primary cancer cells have been shown to overexpress IDO1. In addition, TDO has recently been implicated in human brain tumors.
The earliest experiments had proposed an anti-microbial role for IDO1, and suggested that localized depletion of tryptophan by IDO1 led to microbial death (Yoshida et al., Proc. Natl. Acad. Sci. USA, 1978, 75(8):3998-4000). Subsequent research led to the discovery of a more complex role for IDO1 in immune suppression, best exemplified in the case of maternal tolerance towards the allogeneic fetus where IDO1 plays an immunosuppressive role in preventing fetal rejection from the uterus. Pregnant mice dosed with a specific IDO1 inhibitor rapidly reject allogeneic fetuses through induction of T cells (Munn et al., Science, 1998, 281(5380): 1191-3). Studies since then have established IDO1 as a regulator of certain disorders of the immune system and have discovered that it plays a role in the ability of transplanted tissues to survive in new hosts (Radu et al., Plast. Reconstr. Surg., 2007 June, 119(7):2023-8). It is believed that increased IDO1 activity resulting in elevated kynurenine pathway metabolites causes peripheral and ultimately, systemic immune tolerance. In-vitro studies suggest that the proliferation and function of lymphocytes are exquisitely sensitive to kynurenines (Fallarino et al., Cell Death and Differentiation, 2002, 9(10):1069-1077). The expression of IDO1 by activated dendritic cells suppresses immune response by mechanisms that include inducing cell cycle arrest in T lymphocytes, down regulation of the T lymphocyte cell receptor (TCR) and activation of regulatory T cells (T-regs) (Terness et al., J. Exp. Med., 2002, 196(4):447-457; Fallarino et al., J. Immunol., 2006, 176(11):6752-6761).
IDO1 is induced chronically by HIV infection and in turn increases regulatory T cells leading to immunosuppression in patients (Sci. Transl. Med., 2010; 2). It has been recently shown that IDO1 inhibition can enhance the level of virus specific T cells and concomitantly reduce the number of virus infected macrophages in a mouse model of HIV (Potula et al., 2005, Blood, 106(7):2382-2390). IDO1 activity has also been implicated in other parasitic infections. Elevated activity of IDO1 in mouse malaria models has also been shown to be abolished by in vivo IDO1 inhibition (Tetsutani K., et al., Parasitology. 2007 7:923-30.
More recently, numerous reports published by a number of different groups have focused on the ability of tumors to create a tolerogenic environment suitable for survival, growth and metastasis by activating IDO1 (Prendergast, Nature, 2011, 478(7368):192-4). Studies of tumor resistance have shown that cells expressing IDO1 can increase the number of regulatory T cells and suppress cytotoxic T cell responses thus allowing immune escape and promoting tumor tolerance.
Kynurenine pathway and IDO1 are also believed to play a role in maternal tolerance and immunosuppressive process to prevent fetal rejection in utero (Munn et al., Science, 1998, 281(5380):1191-1193). Pregnant mice dosed with a specific IDO1 inhibitor rapidly reject allogeneic fetuses through suppression of T cells activity (Munn et al., Science, 1998, 281(5380):1191-1193). Studies since then have established IDO1 as a regulator of immune-mediated disorders and suggest that it plays a role in the ability of transplanted tissues to survive in new hosts (Radu et al., Plast. Reconstr. Surg., 2007 June, 119(7):2023-8).
The local immunosuppressive properties of the kynurenine pathway and specifically IDO1 and TDO have been implicated in cancer. A large proportion of primary cancer cells overexpress IDO1 and/or TDO (Pilotte et al., Proc. Natl. Acad. Sci. USA, 2012, Vol. 109(7):2497-2502). Several studies have focused on the ability of tumors to create a tolerogenic environment suitable for survival, growth and metastasis by activating IDO1 (Prendergast, Nature, 2011, 478:192-4). Increase in the number of T-regs and suppression of cytotoxic T cell responses associated with dysregulation of the Kynurenine pathway by overexpression of IDO1 and/or TDO appears to result in tumor resistance and promote tumor tolerance.
Data from both clinical and animal studies suggest that inhibiting IDO1 and/or TDO activity could be beneficial for cancer patients and may slow or prevent tumor metastases (Muller et al., Nature Medicine, 2005, 11(3):312-319; Brody et al., Cell Cycle, 2009, 8(12):1930-1934; Witkiewicz et al., Journal of the American College of Surgeons, 2008, 206:849-854; Pilotte et al., Proc. Natl. Acad. Sci. USA, 2012, Vol. 109(7):2497-2502). Genetic ablation of the IDO1 gene in mice (IDO1−/−) resulted in decreased incidence of DMBA-induced premalignant skin papillomas (Muller et al., PNAS, 2008, 105(44):17073-17078). Silencing of IDO1 expression by siRNA or a pharmacological IDO1 inhibitor 1-methyl tryptophan enhanced tumor-specific killing (Clin. Cancer Res., 2009, 15(2). In addition, inhibiting IDO1 in tumor-bearing hosts improved the outcome of conventional chemotherapy at reduced doses (Clin. Cancer Res., 2009, 15(2)). Clinically, the pronounced expression of IDO1 found in several human tumor types has been correlated with negative prognosis and poor survival rate (Zou, Nature Rev. Cancer, 2005, 5:263-274; Zamanakou et al., Immunol. Lett. 2007, 111(2):69-75). Serum from cancer patients has higher kynurenine/tryptophan ratio, a higher number of circulating T-regs, and increased effector T cell apoptosis when compared to serum from healthy volunteers (Suzuki et al., Lung Cancer, 2010, 67:361-365). Reversal of tumoral immune resistance by inhibition of tryptophan 2,3-dioxygenase has been studied by Pilotte et al. (Pilotte et al., Proc. Natl. Acad. Sci. USA, 2012, Vol. 109(7):2497-2502). Thus, decreasing the rate of kynurenine production by inhibiting IDO1 and/or TDO may be beneficial to cancer patients.
IDO1 and IDO2 are implicated in inflammatory diseases. IDO1 knock-out mice don't manifest spontaneous disorders of classical inflammation and existing known small molecule inhibitors of IDO do not elicit generalized inflammatory reactions (Prendergast et al. Curr Med Chem. 2011; 18(15):2257-62). Rather, IDO impairment alleviates disease severity in models of skin cancers promoted by chronic inflammation, inflammation-associated arthritis and allergic airway disease. Moreover, IDO2 is a critical mediator of autoantibody production and inflammatory pathogenesis in autoimmune arthritis. IDO2 knock-out mice have reduced joint inflammation compared to wild-type mice due to decreased pathogenic autoantibodies and Ab-secreting cells (Merlo et al. J. Immunol. (2014) vol. 192(5) 2082-2090). Thus, inhibitors of IDO1 and IDO2 are useful in the treatment of arthritis and other inflammatory diseases.
Kynurenine pathway dysregulation and IDO1 and TDO play an important role in the brain tumors and are implicated in inflammatory response in several neurodegenerative disorders including multiple sclerosis, Parkinson's disease, Alzheimer's disease, stroke, amyotrophic lateral schlerosis, dementia (Kim et al., J. Clin. Invest, 2012, 122(8):2940-2954; Gold et al., J. Neuroinflammation, 2011, 8:17; Parkinson's Disease, 2011, Volume 2011). Immunosuppression induced by IDO1 activity and the Kynurenine metabolites in the brain may be treated with inhibitors of IDO1 and/or TDO. For example, circulating T-reg levels were found to be decreased in patient with glioblastoma treated with anti-viral agent inhibitors of IDO1 (Soderlund, et al., J. Neuroinflammation, 2010, 7:44).
Several studies have found Kynurenine pathway metabolites to be neuroactive and neurotoxic. Neurotoxic kynurenine metabolites are known to increase in the spinal cord of rats with experimental allergic encephalomyelitis (Chiarugi et al., Neuroscience, 2001, 102(3):687-95). The neurotoxic effects of Kynurenine metabolities is exacerbated by increased plasma glucose levels. Additionally, changes in the relative or absolute concentrations of the kynurenines have been found in several neurodegenerative disorders, such as Alzheimer's disease, Huntington's disease and Parkinson's disease, stroke and epilepsy (Németh et al., Central Nervous System Agents in Medicinal Chemistry, 2007, 7:45-56; Wu et al. 2013; PLoS One; 8(4)).
Neuropsychiatric diseases and mood disorders such as depression and schizophrenia are also said to have IDO1 and Kynurenine dysregulation. Tryptophan depletion and deficiency of neurotransmitter 5-hydroxytryptamine (5-HT) leads to depression and anxiety. Increased IDO1 activity decreases the synthesis of 5-HT by reducing the amount of Tryptophan availability for 5-HT synthesis by increasing Tryp catabolism via the kynurenine pathway (Plangar et al. (2012) Neuropsychopharmacol Hung 2012; 14(4): 239-244). Increased IDO1 activity and levels of both kynurenine and kynurenic acid have been found in the brains of deceased schizophrenics (Linderholm et al., Schizophrenia Bulletin (2012) 38: 426-432)). Thus, inhibition of IDO1, IDO1, and TDO may also be an important treatment strategy for patients with neurological or neuropsychiatric disease or disorders such as depression and schizophrenia as well as insomnia.
Kynurenine pathway dysregulation and IDO1 and/or TDO activity also correlate with cardiovascular risk factors, and kynurenines and IDO1 are markers for Atherosclerosis and other cardiovascular heart diseases such as coronary artery disease (Platten et al., Science, 2005, 310(5749):850-5, Wirlietner et al. Eur J Clin Invest. 2003 July; 33(7):550-4) in addition to kidney disease. The kynurenines are associated with oxidative stress, inflammation and the prevalence of cardiovascular disease in patients with end-stage renal disease (Pawlak et al., Atherosclerosis, 2009, (204)1:309-314). Studies show that kynurenine pathway metabolites are associated with endothelial dysfunction markers in the patients with chronic kidney disease (Pawlak et al., Advances in Medical Sciences, 2010, 55(2):196-203).
There is a need in the art for compounds that are inhibitors of the indoleamine 2,3-dioxygenase-1 and/or indoleamine 2,3-dioxygenase-2 and/or tryptophan 2,3-dioxygenase pathway, as well as for methods for treating diseases that can benefit from such inhibition.